1. Field of the Invention
This application relates generally to engines and power generation systems that use high viscosity, high auto ignition temperature liquid fuels for generating power and heat. More particularly, it relates to an open cycle air engine where regenerative heating is used to preheat and vaporize glycerin, and where the combustion inlet air is also preheated by the afterburning power generation cycle, so that it the glycerin fuel may be cleanly and completely combusted to provide heat energy to operate the engine.
2. Description of Prior Art
Glycerin, C3H5(OH)3, is also known by other names including glycerol; glycerine; and 1,2,3-Propanetriol. It is widely used in industry for applications such as paints, antifreeze, pharmaceuticals, and cosmetics. Typically, glycerin has been manufactured as a byproduct of soap or from processing propylene. However, recently, very large quantities of glycerin have become available as a byproduct of the production of biodiesel fuel. The biodiesel production process generates a gallon of glycerin for approximately every 10 gallons of biodiesel. As biodiesel production has grown, the glycerin market has become saturated. Instead of receiving the anticipated revenue from selling the glycerin byproduct to chemical companies, biodiesel manufacturers are now having to pay for disposal of what has become an unwanted waste product.
Ideally, the waste glycerin could be combusted to produce heat for the biodiesel production process, making it a self fueling process; much like 19th century whaling ships burned waste portions of whales as they rendered blubber for oil. Unfortunately, glycerin is very difficult to burn for three reasons: First, it has a high viscosity and that makes it difficult to inject into a combustion process. Second, it has a high auto ignition temperature (393 C. or 739 F.) that makes ignition difficult to initiate and sustain. Finally, it begins to decompose to form toxic acrolein at temperatures above 150 C. or 302 F. For these reasons, the biodiesel industry does not currently use glycerin to fuel its process.
The following table illustrates the key property differences between gasoline and glycerin that make glycerin combustion difficult. Glycerin's viscosity is orders
Comparison of Key Combustion PropertiesGasolineGlycerinViscosity @ 20 C. (c.p.)0.61499Normal Boiling Point (° C.)~126290Auto-Ignition Temp (° C.)280393Heat of Vaporization (cal/g)75229Heat of Combustion (cal/g)115004144of magnitude larger than gasoline, making it much more difficult to effectively force through injectors and nozzles. Glycerin requires significantly higher temperatures and energies to vaporize and auto ignite while releasing much lower specific energy from the combustion process.
The difficulties of glycerin combustion have similarly made it unsuitable for an engine fuel. It cannot be burned in an internal combustion engine and has previously been impractical for fueling turbine engines.
Nevertheless, the prior-art contains several means for combusting glycerin and similar high viscosity, high auto ignition temperature fuels.
U.S. Pat. No. 4,188,782 (“Fuel Vaporizing Combustor Tube”, Smith et al., 1980) and U.S. Pat. No. 4,242,863 (“Dual Fuel Vaporizing Combustor”, Baily, 1981) teach methods for combusting high viscosity fuels by vaporizing, or partially vaporizing, the fuel with preheated air and then impinging the fuel/air mix onto a hot surface plate located in the combustion zone.
U.S. Pat. No. 4,838,029 (Externally Vaporizing System for Turbine Combustor”, Gleason, deceased et al., 1989) teaches a gas turbine combustor where compressor bleed air is mixed with fuel in an auxiliary burner to generate “very hot, nearly-inert gases for vaporizing the main fuel supply” and then injecting the combined hot gases and vaporized fuel into the combustion chamber where it mixes with additional air to complete the combustion.
A promising prior-art is contained in US 2008/0305445 A1 (“Process for Combustion of High Viscosity Low Heating Value Liquid Fuels”, Roberts et al. 2008). Roberts teaches preheating the glycerin to reduce its viscosity and then atomizing it with high pressure (190 psig) air.
The prior-art methods for combusting glycerin typically are based on a process of preheating the liquid fuel to reduce viscosity and then mixing the preheated fuel with air or combustion products to further atomize or vaporize the fuel for complete combustion. All these methods require fairly complex and expensive burners. In addition, Gleason's and Roberts' methods require a source of high pressure air.
A much simpler means of combusting glycerin and similar fuels is possible for the specific case where the fuel is intended for use in an Indirectly Fired Gas Turbine (IFGT) or Afterburning, Recuperated, Positive Displacement (ARPD) engine (U.S. Pat. No. 7,028,476B2 “Afterburning, Recuperated, Positive Displacement Engine”, Proeschel, 2006).
Both the IFGT and ARPD engines operate as shown in FIG. 1. Both are air cycle engines utilizing the steps of air compression, indirect heating of the compressed air, expansion of the hot air to produce power, exhausting of the expanded air into a burner to support combustion, and finally using the combustion products to provide heat to the compressed air. Both the IFGT and ARPD can be considered “afterburning engines” because the combustion process takes place after the air has done mechanical work in the engine. In an IFGT the “Compressor” and “Expander” shown in FIG. 1 are turbomachines. In an ARPD the “Compressor” and “Expander” shown in FIG. 1 are positive displacement machines.
The primary aim of the glycerin combustion process for afterburner fired powerplants is to overcome the disadvantages of the prior art by using the unique configuration of the IFGT and ARPD family of engines to provide an integrated system to completely and cleanly combust glycerin or similar fuels to achieve an economical and efficient means of producing electric and/or mechanical power.